I've got a Lastolite pop up soft box that I really rate - perfect for single portraits and can be used for small groups (three max really). It's small enough that it won't blow over too easily and it folds down to the size of a small reflector. It mounts onto a light stand.
Other than that I've found that umbrellas are an easy way to soft light a scene - although the light control can be quite poor. CotswoldsPhoto on eBay seem to have a good selection. The plus side is that they're cheap, the downside is that they'll blow over if you sneeze!